1. Field of the Invention
This invention relates to disk head assemblies for supporting read/write heads adjacent rotating disks in disk drives and more particularly, to a swage mount for attaching a head suspension assembly to a head actuator arm.
2. Description of the Prior Art
In hard disk drives data are stored on magnetizable surfaces of a plurality of rotatable disks that are mounted in a coaxial stack on a housing of the drive. Transducer heads that write data to and read data from the disk surfaces are supported by an actuator that is mounted on the housing and can be actuated to position the transducer heads in alignment with concentric data tracks defined on the disks. Each transducer head is attached to one end of a head suspension that is connected to an actuator arm that extends from the actuator body. The suspensions include a flexible load beam constructed of light sheet steel that has a bend formed in it. The load beam acts as a spring that forces the head against the disk surface with an accurate preload or “gram load”. Air turbulence caused by the rotating disks lifts the heads slightly off of the disks so that the heads fly on an air bearing across the disk surfaces. The suspension gram load counteracts the air bearing force.
The head suspension is attached to the actuator arm using a base plate (swage mount) that forms a part of the head suspension. The base plate includes a flat flange portion and a cylindrical hub portion or boss. The base plate hub is passed through a load beam clearance hole in the load beam and the flange is spot welded to the load beam. The combined base plate, load beam and a flexure make up a head suspension, and the suspension has the hub of the base plate extending through and beyond the load beam clearance hole.
The hubs of two suspensions are inserted into an actuator arm boss hole formed through an actuator arm extending from an actuator body, one hub entering an actuator arm boss hole from each end of the hole. A swage ball is passed through the cylindrical hubs to force the peripheries of the hubs to expand (swage) into tight engagement with the inner peripheries of the actuator arm boss hole. Thus, an actuator arm may carry two suspensions on opposite sides thereof to support two transducer heads in opposing directions, one up and one down.
The hub is an extended boss whose upper end is contiguous with the flange portion and whose lower end passes through boss clearance holes in both the load beam and the actuator arm. The hub region supplies the retention torque between the base plate, which is welded to the load beam, and the actuator arm by plastically expanding and cold working during the swaging operation, creating a press fit with the actuator arm.
Problems with this method of mounting transducer heads have arisen as the need for increased data storage capacity in hard disk drives has grown and the size of the disk drive has decreased to fit in small lap top computers. The problem of forming a strong connection between the actuator arms and the transducer suspensions has been made more difficult as the size of the components has become smaller. In the prior art, relatively high swaging forces are needed to insure that a base plate makes a strong connection with the actuator arm boss hole. As the part get smaller and thinner, these high forces cause unacceptable large distortions in the load beam and cause preload changes.
One such method for reducing the overall drive size is to reduce the size of the stacked vertical joint connecting the load beam to the actuator assembly. For example, in U.S. Pat. Nos. 6,183,841 and 5,689,389, a low profile swage mount fastener is used to connect a load beam to an actuator arm of an actuator assembly. Because the swage mount fastener has a low profile, the overall height of the disc drive may be reduced. However, a disadvantage of using a low profile swage mount fastener is that as performance demands increase, it provides less torque retention than is required to withstand the force of the load beam.
In U.S. Pat. Nos. 6,183,841 and 5,689,389 the torque retention characteristics of a low profile swage mount fastener were increased by modifying the internal geometry of the swage mount. However, the level of torque retention that can be achieved solely by modifying the swage mount design is limited. Without increased torque retention values, the acceleration rate a load beam can withstand is limited, which imposes an upper limit on the speed at which the read/write head can be positioned. This in turn will limit the overall access time a disc drive can achieve, a key parameter of disc drive performance. Accordingly, there is a need for a low profile swage mount fastener capable of torque retention values that are significantly higher than have been previously achieved.
U.S. Pat. Nos. 6,231,689 and 6,351,349 are directed at overcoming shortcomings of the prior art. Each patent provides a surface hardened swage mount, a method of hardening the metal from which such parts are made, and for connecting a disc drive actuator arm to a load beam, which results in an increased torque retention characteristic of the swage mount.
Currently, the industry is moving toward nickel-plated aluminum actuator arms, because nickel-plated aluminum arms provide much improved overall cleanliness.
As described above, as base plates get smaller to accommodate the geometries of smaller disk drives, reduced retention torque becomes a problem and this has given rise to a need to increase retention torque. The need to increase retention torque is a problem especially with nickel-plated aluminum actuator arms. A base plate is disclosed in copending U.S. patent application Ser. No. 10/037,643 in which the outer surface of the hub includes numerous protrusions that are less than approximately 50 microns in height. The protrusions are primarily composed of a material (such as a carbide or a nitride) which is different from the stainless steel hub. Preferably, the protrusions are substantially harder (such as at least 50 hardness Vickers harder) than the base material of the hub. The purpose of the protrusions is to provide greater torque retention when the base plate is swaged to an actuator arm.
During manufacture, chromium carbide or chromium nitride is precipitated out of a base metal onto the outer surface of the hub resulting in the surface protrusions. The surface protrusions stick out of the hub outer surface and grab into the aluminum actuator arm boss hole when the hub is swaged. These and other methods of creating hardened modules on the outer hub surface can boost retention torque by 60%-100%.
Swage mounts containing carbides provide higher retention torque than nitrided parts, but tend to shed a higher volume of particles from the surface during the swaging process. Due to the present emphasis on cleanliness within the industry, this currently limits the use of nitride, the most effective precipitate.
It is desirable in swage mount manufacture to increase retention torque and yet maintain or improve cleanliness levels, which may or may not be in conjunction with the aforementioned surface protrusions.
Further, it is also desirable to increase torque in both Al and Ni-plated Al actuator arms.
During current manufacturing of base plates, the base plates are subjected to processes that remove burrs, which may include tumbling using porcelain beads. The reason the base plate is deburred is that a burr may flake off and contaminate the drive mechanism. Furthermore, a burr can cause the base plate to stand off and not mate with the load beam properly. Tumbling to deburr the base plate using porcelain beads that are predominately aluminum oxide may result in aluminum oxide particles coming loose and becoming embedded in the surface of the disk. Studies of failed disk drives have shown that aluminum oxide separating from the beads has been found on the disk surface at the site of a head crash. Even if the head does not crash, an aluminum oxide particle embedded on the disk can cause a thermal asperity. As the head passes over the asperity, the head may be damaged by heat from the friction or an inaccurate reading may occur.
Manufacture of the material used to fabricate the base plates, typically stainless steel, often results in the introduction of metal oxides such as alumina and magnesia into the melt. These oxides and other contaminants in the base metal can form inclusions that may potentially be exposed at the surface of the base plate after manufacturing. These inclusions, should they become loose and fall from the base material, can pose a threat to drive operation in the form of a head crash or thermal asperity, as described above.
It is therefore desirable to cover the imbedded particles and material inclusions, to prevent them from coming loose from the swage mounts during service.